1. Field of the Invention
The present invention relates to a semiconductor light emitting device, such as a light emitting diode device, particularly having a semiconductor light emitting element which generates a wavelength-converted light to emit the light having its wavelengths of 550 nm or less to the outside.
2. Description of the Prior Art
Use of a semiconductor light emitting element having a large energy gap allows realization of a semiconductor light emitting device which generates fight at relatively short wavelengths from visible light of short wavelengths to ultraviolet light. A semiconductor light emitting element for generating light having such wavelengths, includes nitrogen gallium compound semiconductors, such as GaN, GaAlN, InGaN and InGaAlN which can be utilized to provide a new solid-state ultraviolet light source to offer a variety of advantages including small size, low power consumption, and long life.
FIG. 4 shows a sectional view of a prior art light emitting diode device 1 which utilizes a fluorescent substance 7a for converting the wavelength of the light projected from a light emitting diode chip 2. As shown in FIG. 4, the light emitting diode chip 2 is secured to a bottom surface 3b of a concavity 3a formed in a first external terminal 3 as a cathode lead. A cathode electrode 2g formed on the light emitting diode chip 2 is connected to a first wire connection 9a of the first external terminal 3 by means of a first lead wire 5. Also, an anode electrode 2f formed on the light emitting diode chip 2 is connected to a second wire connection 9b of a second external terminal 4 as an anode lead by means of a second lead wire 6. The light emitting diode chip 2 secured on the concavity 3a is covered with a light permeable protective plastic material 7 filled in the concavity 3a with fluorescent substance 7a blended in the plastic material 7. Moreover, a light permeable plastic encapsulant 8 encapsulates the light emitting diode chip 2, concavity 3a and first and second wire connections 9a, 9b of the first and second external terminals 3 and 4, and lead wires 5, 6.
When a voltage is applied across the first and second external terminals 3 and 4 of the light emitting diode device 1, the light emitting diode chip 2 is activated to generate a light which passes through the protective plastic material 7, and then is reflected on a side wall 3c of the concavity 3a formed in the first external terminal 3. Subsequently, the light passes through the transparent plastic encapsulant 8 and is then released out of the light emitting diode device 1. Some of the light is projected from a top of the light emitting diode chip 2, and is directly passed through the protective material 7 and plastic encapsulant 8 without the reflection on the side wall 3c of the concavity 3a before the release to the outside of the light emitting diode device 1. The plastic encapsulant 8 is formed with a lens portion 8a at the top so that the lens portion 8a converges the light passed through the plastic encapsulant 8 for increased directivity. Upon passing through the protective plastic material 7, the light projected from the light emitting diode chip 2 is converted into the light of different wavelength by the fluorescent substance 7a mixed into the protective plastic 7 before the release. Accordingly, the light emitting diode device 1 releases light different in wavelength from the light projected out of the light emitting diode chip 2.
Generally, a semiconductor light emitting element is covered with a plastic sealer or plastic encapsulant 8 which comprises an organic polymeric compound in which elements such as carbon, hydrogen, oxygen and nitrogen are linked together in the mesh form. In this case, it is known that the bonds of the organic polymer are cut, when the plastic sealer or epoxy resin encapsulant 8 of the diode device is irradiated with ultraviolet light or the like, resulting in deterioration of various optical or chemical characteristics of the plastic sealer. For example, a GaN (gallium nitride) light emitting diode chip generates ultraviolet light having wavelengths up to approximately 365 nm, and therefore, when irradiated with ultraviolet light, the plastic sealer 8 is gradually yellowed or discolored, starting from the periphery with the highest light intensity of the light emitting diode chip 2. Accordingly, the visible light generated by the light emitting diode chip 2 is absorbed by the colored area, causing the light to be decayed. Also, the discoloration results in the deterioration, reduced moisture resistance and increased ion permeability of the plastic sealer 8 so that contamination or undesirable ions can enter from outside through the plastic sealer 8 into the light emitting diode chip 2, which results in the deterioration of the diode chip 2 itself and synergistically decreased intensity of emission light for the light emitting diode device 1.
In addition, when a light emitting diode chip of GaN (gallium nitride) has a high forward voltage, it generates the high power loss even with a relatively low forward current, and the considerably elevated temperature in the diode chip 2 in operation. It is generally known that a plastic material is gradually deteriorated to be yellowed or discolored when subjected to heat or a high temperature. Therefore, if a GaN light emitting diode chip is assembled to prepare a prior art light emitting diode device, the plastic material is gradually yellowed or discolored, starting from the area contacting with the high-temperature light emitting diode chip, as light of short wavelength is projected from the light emitting diode chip, thereby causing the degradation in quality of the appearance and gradually lowered emission light intensity of the light emitting diode device. Thus, the prior art light emitting diode device causes a limited and reduced number of selectable types of material, a decrease in reliability, imperfection of light conversion function and a rise in product price.
In view of the above-mentioned fact that the ultraviolet light causes the debasement of the plastic sealer for a short period of time with the reduced light emission efficiency, a hermetically sealed structure has been proposed for some of the light emitting devices which include an outer container for hermetically sealing the semiconductor light emitting element in the container to completely insulate it from the external atmosphere, and an inactive or stable sealing gas such as nitrogen filled in the container.
Although the hermetically sealed structure does not cause the deterioration of the plastic sealer, it raises a problem of the final expensive products because it requires costly materials and a relatively complicated process for manufacture. In addition, because the container is filled with an inactive gas of its refractive index greatly different from that of the gallium nitride compound semiconductor, disadvantageously a reflection plane is formed at the boundary between the gallium nitride compound semiconductor and the inactive gas. Consequently, a defect is presented in that the light projected from the semiconductor light emitting element is repetitively reflected at the boundary between the gallium nitride compound semiconductor and the inactive gas, resulting in the decayed or attenuated light and the lowered light emission efficiency.
Further, various problems in practical use have been presented with the conventional light emitting diode device 1 which comprises the light emitting diode chip 2 surrounded by the protective plastic material 7 with the fluorescent substance 7a contained therein, and the plastic encapsulant 8 for surrounding the whole of the diode chip 2 and plastic material 7. Firstly, when the protective plastic 7 and the plastic encapsulant 8 do not always have a sufficient environmental resistance, only a specific type of the fluorescent substance 7a can be compounded into the protective plastic material 7. Generally, a plastic material is permeable to moisture which permeates the plastic material with the lapse of time, when it is left in a high humidity atmosphere. In this case, the permeating moisture may cause decomposition or change in quality of some fluorescent substances if they have their poor resistance to moisture resulting in the reduction or loss of the light wavelength conversion capability. For example, the well-known typical fluorescent substance 7a of calcium sulfide cannot be used with the conventional light emitting diode device 1 because it is hydrolyzed by moisture.
In addition, not only moisture but also foreign matter ions, such as sodium and chlorine, permeate the plastic material, and have a harmful effect on the light emitting diode chip 2. Therefore, although the light emitting diode device 1 is manufactured in a clean environment, if it is left in an atmosphere containing foreign matter ions, it would present a serious problem that those ions gradually permeate the inside of the plastic material resulting in the deteriorated electrical characteristics of the light emitting diode chip 2. Particularly, it is a critical problem that not a few of the organic fluorescent substances available are chemically unstable due to presence of undesirable and harmful free foreign matter ions. Therefore, the conventional light emitting diode device 1 cannot utilize such organic fluorescent substances.
Another problem is that the plastic encapsulant is debased by irradiation of the short-wavelength light, such as ultraviolet light generated from the light emitting diode chip 2. As above-mentioned, since the protective plastic material 7 and the plastic sealer 8 comprises an organic polymeric compound of elements such as hydrogen, oxygen and nitrogen linked together in the mesh form, it is known that the bonds of the organic polymeric compound are cut when ultraviolet light is projected on them, which results in deterioration of the various optical and chemical characteristics. For example, a blue light emitting diode chip of GaN (gallium nitride) may have an emission light component in the ultraviolet wavelength region of 380 nm or less, in addition to the visible light component so that the plastic encapsulant is gradually yellowed or discolored, starting from the periphery with the high light intensity of the light emitting diode chip, and thus, the visible light generated by the light emitting diode chip would be decayed due to absorption by the colored area. Further, the deterioration of the plastic encapsulant causes the lowered resistance to moisture and the increased ion permeability to thereby damage the light emitting diode chip 2 and synergistically decrease the emission light intensity of the light emitting diode device 1.
Still further, there are great restrictions on selection of the fluorescent substance and the light emission characteristics of the light emitting diode device because it cannot make use of a light emitting diode chip for generating ultraviolet light, and this is the third problem. Many developments and improvements have been made since old days on the fluorescent substances excited by the ultraviolet light for use in a fluorescent or mercury lamp or the like so that a number of fluorescent substances are put to practical use at present because they have a variety of wavelength distributions of emission light, and are inexpensive and high in light conversion efficiency. If a diode chip for emitting ultraviolet light can be combined with fluorescent substances excited by the ultraviolet light, it would be expected to establish light emitting diode devices which are still more brighter and greatly vary in color tone. However, the conventional light emitting diode device cannot utilize ultraviolet light emitting diode chips because the plastic materials and fluorescent substance may be deteriorated by the ultraviolet light although the fluorescent substances are excellent in light conversion efficiency.
The fourth problem is that the light projected from the light emitting diode chip is decayed when passed through the plastic encapsulant which is yellowed or discolored because of its low heat resistance. As stated above, the GaN (gallium nitride) blue light emitting diode chip of a type for example with its high forward voltage, produces the high power loss even with a relatively low forward current, and the chip temperature is considerably raised in service. It is known that a plastic material is gradually deteriorated to be yellowed or discolored when heated to a high temperature. Therefore, if a GaN light emitting diode chip is used with a conventional light emitting diode device, the plastic material is gradually yellowed or discolored, starting from the area contacting with the light emitting diode chip of elevated temperature, thereby causing the gradual deteriorated quality in appearance and the reduced emission light intensity of the light emitting diode device 1. In this way, in the conventional light emitting diode device, the above-mentioned problems would be presented by compounding a fluorescent substance into the plastic material, resulting in a reduced number of selectable types of materials, a decrease in reliability, imperfection of light conversion function, and a rise in product price.
An object of the present invention is to provide a semiconductor light emitting device which has excellent resistances to environment and ultraviolet light.
Another object of the present invention is to provide a semiconductor light emitting device resistible to heat.
The semiconductor light emitting device according to the present invention comprises a base (3, 4, 11), a semiconductor light emitting element (2) secured to the base (3, 4, 11), and a coating material (10) for covering the semiconductor light emitting element (2) wherein the coating material (10) is a light permeable polymetaloxane or ceramic formed from a metal alcoxide, a ceramic precursor polymer or the like. Unlike the organic plastic material, the coating material (10) will not be deteriorated under an elevated temperature environment in which ultraviolet light is projected thereon over a long period of time because the coating material (10) is a polymetaloxane or a ceramic which offers resistances to ultraviolet light and heat when irradiated with such as ultraviolet light of short-wavelength.
In an embodiment of the present invention, the coating material (10) is in the state of highly pure glass and does not have an ill effect on the characteristics of the semiconductor light emitting element (2) because the coating material (10) has an extremely small amount of impurities, compared to low-melting point glass, etc. containing boron, lead oxide, etc. Further, the coating material (10) is in the state of highly heat-resistant glass which, therefore, does not cause reduction in light permeability due to yellowing and the like. The coating material comprises the glass formed based on the metaloxane bond, or a ceramic formed from a ceramic precursor.
The semiconductor light emitting device is prepared by securing the semiconductor light emitting element (2) to the base (3, 4, 11); applying on the semiconductor light emitting element (2) a filler of the polymetaloxane sol obtained from the metal alcoxide or the ceramic precursor polymer; and then, drying and heat treating the filler to form the coating material (10). Because the coating material (10) is formed by the sol-gel technique for metal alcoxide or from the ceramic precursor polymer, it is vitrified at a low temperature to provide a transparent or clear noncrystalline metal oxide.
With use of a metal alcoxide, a type of organic metal compound as a starting material in the sol-gel technique, the solution is hydrolyzed and polymerized with condensation to form a sol, and further the reaction is advanced by moisture in air etc. for gelation to obtain a solid metal oxide. For example, when tetraethoxysilane as a metal alcoxide of silicon is used in the process for forming a silica glass membrane, the tetraethoxysilane is dissolved in a solvent such as alcohol; a catalyst such as an acid and a small amount of water are added to the mixture; and the solution is thoroughly agitated to form a polysiloxane sol in the liquid state in accordance with the following reaction formulae:
Hydrolysis reaction: Si(OC2H5)4+4H2Oxe2x86x92Si(OH)4+4C2H5OH
Dehydration-condensation reaction: nSi(OH)4xe2x86x92[SiO2]n+2nH2O
A number of SiO2 (silica) molecules of the polysiloxane sol generated as a result of the above-mentioned reactions are bonded to one another to form a polymer, and the fine particles of this polymer are dispersed in the alcohol solution. When the polysiloxane 801 is applied to the base (3, 4, 11) and dried, the volume of the sot contracts or shrinks because the solvent, water and ethyl alcohol produced by the reaction are evaporated, and as a result the residual OH groups at the ends of adjacent polymers are bonded to each other due to the dehydration reaction with condensation to cause the coating to be gelled or solidified. Subsequently, the gel coating obtained is baked to strengthen the bonds between the polysiloxane particles and finally form a gel coating having a high mechanical strength.
The coating material (10) has a permeability to the light projected from the semiconductor light emitting element (2), and contains a fluorescent substance (10a) which absorbs the light projected from the semiconductor light emitting element (2), and converts it into light different in emission light wavelength. The coating material (10) is formed by baking the coating agent comprising a metal alcoxide, such as tetramethoxysilane and tetraethoxysilane, or a ceramic precursor polymer, such as perhydropoly-silazane, and tightly and strongly adheres to the semiconductor light emitting element (2) and the external terminals (3, 4). The light emitted from the semiconductor light emitting element (2) can be converted into light having a desired emission light wavelength by means of the fluorescent substance (10a) in the coating material (10) for surrounding the semiconductor light emitting element (2), and the light is released to the outside through the coating material (10).
The semiconductor light emitting element (2) is secured to the base (3, 4, 11) through the adhesive (12) formed of an organic resin or a polymetaloxane. Particularly, the adhesive formed of a polymetaloxane is hardly deteriorated when irradiated with short-wavelength light such as ultraviolet light.
A gallium nitride semiconductor light emitting element (2) efficiently generates light at short wavelengths of 365 nm to 650 nm to form a to semiconductor light emitting device with the high emission light luminance and high reliability. Since short-wavelength light tends to particularly deteriorate prior art plastic coating material (7) and plastic adhesive, the present invention would give rise to a great effect for preventing deterioration of the coating material (10) and adhesive (12). The fluorescent substance (10a) absorbs a part of the light from the semiconductor light emitting element (2), and converts the wavelength from short to long at a high light-conversion efficiency. The base (3, 4, 11) comprises first and second external terminals (3, 4), and the semiconductor light emitting element (2) comprises the electrodes (2f, 2g) electrically connected respectively to the first and second external terminals (3, 4).
The coating material (10) which is permeable to the light projected from the semiconductor light emitting element (2) covers the semiconductor light emitting element (2), and the end portions of the first external terminal (3) and the second external terminal (4) in the vicinity of the semiconductor light emitting element (2). The coating material (10) is formed by solidifying a coating agent which comprises a solution produced by hydrolyzing and polymerizing a metal alcoxide by the sol-gel technique, a solution containing a ceramic precursor polymer, or a combination of these solutions.
Since the coating material (10) has resistances to ultraviolet light and heat, the utilization of the coating material (10) can surely prevent yellowing and discoloring of the coating material (10) itself and the encapsulant (8) for covering the coating material (10), and also prevent the optical characteristics of the semiconductor light emitting device from being deteriorated, so that the resistance to environment can be maintained by the double covering structure of the encapsulant (8) and the coating material (10).
In an embodiment of the present invention, the coating material (10) is formed by drying and baking the coating agent to tightly and strongly adhere to the semiconductor light emitting element (2). Formed in either end portion of the first and second external terminals (3, 4), is a concavity (3a) with the bottom (3b) on which the semiconductor light emitting element (2) is secured together with the coating material (10).
The metal alcoxide is selected from the silicontetra alcoxides, such as Si(OCH3)4, Si(OC2H5)4, Si(i-OC3H7)4, and Si(t-OC4H9)4, the single metal alcoxides, such as ZrSi(OCH3)4, Zr(OC2H5)4, Zr(OC3H7)4, Si(OC4H9)4, Al(OCH3)3, Al(OC2H5)3, Al(iso-OC3H7)3, Al(OC4H9)3, Ti(OCH3)4, Ti(OC2H5)4, Ti(iso-OC3H7)4, and Ti(OC4H9)4, the two-metal alcoxides, such as La[Al(iso-OC3H7)4]3, Mg[Al(iso-OC3H7)4]2, Mg[Al(sec-OC4H9)4]2, Ni[Al(iso-OC3H7)4]2, Ba[Zr2(C2H5)9]2, and (OC3H7)2Zr[Al(OC3H7)4]2, and the multi-metal alcoxides. The ceramic precursor polymer is perhydropolysilazane. The coating material (10) is formed by baking the metal alcoxide or the ceramic precursor polymer at a temperature lower than the melting point of the semiconductor light emitting element (2). The coating material (10) is a clear or transparent coating layer for example a solid glass layer based on the metaloxane bond. The metal alcoxide is expressed by the general formula M(OR)n, where M is at least one type of metal selected from the group comprising silicon (Si), aluminum (Al), zirconium (Zr), and titanium (Ti); R is a homogeneous or heterogeneous saturated or unsaturated aliphatic hydrocarbon group having 1 to 22 carbon atoms; and n is the number equivalent to the metal valency.
The electrodes (2f, 2g) formed on the top of the semiconductor light emitting element (2) are electrically connected respectively to the first and second external terminals (3, 4) by means of the first and second lead wires (5, 6); the coating material (10) covers the semiconductor light emitting element (2), the electrodes (2f, 2g) and the end portions of the first and second lead wires (5, 6) connected to the electrodes (2f, 2g) of the semiconductor light emitting element (2); and the coating material (10) tightly and strongly adheres to the end portions of the first and second lead wires (5, 6).
In another embodiment of the present invention, a concavity (3a) is formed in one principal surface of the insulative substrate (11) for constituting the base (3, 4, 11); the first and second external terminals (3, 4) are formed to extend in the directions opposite to each other along the one principal surface of the insulative substrate (11); and the semiconductor light emitting element (2) is secured to either of the first and second external terminals (3, 4) at the bottom (3b) of the concavity (3a). The first and second external terminals (3, 4) extend from one principal surface of the insulative substrate (11) to the other principal surface along the sides.
The coating material (10) is formed not to extend from the upper end of the concavity (3a) in order to prevent false lighting of a deactivated semiconductor light emitting device adjacent to an activated semiconductor light emitting device. The light projected from the semiconductor light emitting element (2) is passed through the coating material (10) and the encapsulant (8) of plastic material, and thereafter is released out of the encapsulant (8). As the light projected from the semiconductor light emitting element (2) reaches the fluorescent substance (10a) in the glass layer of the coating material (10), the light component is subjected to wavelength-conversion in the coating material (10) so that the converted light is mixed with the light component without wavelength-conversion from the semiconductor light emitting element (2) to give off the mixed light to the outside through the plastic encapsulant (8).
An additive or additives may be compounded into the coating material (10) such as a light absorption substance for absorbing emitted light having a specific wavelength, a light scattering substance (10b) for scattering the light emitted from the semiconductor light emitting element (2) or a binder (10b) for preventing the coating material (10) from being cracked.
The method for manufacturing the semiconductor light emitting device according to the present invention comprises the steps of forming a concavity (3a) in the base (3, 4, 11); securing the semiconductor light emitting element (2) to the bottom (3b) of the concavity (3a) and electrically connecting the electrodes (2f, 2g) formed on the semiconductor light emitting element (2) to the first and second external terminals (3, 4); pouring the coating agent into the concavity (3a) for covering the semiconductor light emitting element (2), the electrodes (2f, 2g) and the end portions of the first and second lead wires (5, 6) connected to the electrodes (2f, 2g), the coating agent having a permeability to the light projected from the semiconductor light emitting element (2), and the coating agent comprising a solution produced by hydrolyzing and polymerizing a metal alcoxide by the sol-gel technique, a solution containing a ceramic precursor polymer or a combination of these solutions; and finally baking the coating agent to form the coating material (10) for covering the semiconductor light emitting element (2).
The embodiments of the present invention may comprise any one of the steps of pouring the solution containing a metal alcoxide into the concavity (3a); forming the base (11) of an insulative substrate; sealing the coating material (10) with the encapsulant (8); tightly and strongly applying the coating material (10) to the semiconductor light emitting element (2) and the first and second external terminals (3, 4); forming a concavity (3a) in either end portion of the first and second external terminals (3, 4) used as the base (3, 4, 11); or baking the coating agent at a temperature lower than the melting point of the semiconductor light emitting element (2) to form the coating material (10). Also, the method may further comprise forming a concavity (3a) in one principal surface of the insulative substrate (11) as the base (3, 4, 11); and forming the first and second external terminals (3, 4) extending in the directions opposite to each other along the one principal surface of the insulative substrate (11). The method may comprise a process of electrically connecting the electrodes (2f, 2g) of the semiconductor light emitting element (2) to the first and second external terminals (3, 4) by means of the first and second lead wires (5, 6).
One embodiment of the present invention comprises the process of forming a concavity (3a) in either end portion of the first and second external terminals (3, 4); attaching the semiconductor light emitting element (2) to the bottom (3b) of the concavity (3a); electrically connecting the electrodes (2f, 2g) formed on the semiconductor light emitting element (2) to the first and second external terminals (3, 4) by means of the lead wires (5, 6); pouring the coating agent into the concavity (3a) to cover the semiconductor light emitting element (2), the electrodes (2f, 2g) and the end portions of the lead wires (5, 6) connected to the electrodes (2f, 2g), the coating agent having a permeability to the light projected from the semiconductor light emitting element (2), the coating agent comprising a metal alcoxide or a ceramic precursor polymer and containing a fluorescent substance which absorbs the light projected from the semiconductor light emitting element (2) and converts it into light different in emission light wavelength; and further sealing the coating material (10) with the encapsulant (8), the coating material (10) tightly and strongly adhering to the semiconductor light emitting element (2) and the external terminals (3, 4).
Another embodiment of the present invention comprises the process of forming a concavity (3a) in one principal surface of the insulative substrate (11) for constituting the base (3, 4, 11); forming the first and second external terminals (3, 4) extending in the directions opposite to each other along the one principal surface of the insulative substrate (11); attaching the semiconductor light emitting element (2) to either of the first and second external terminals (3, 4) at the bottom (3b) of the concavity (3a); electrically connecting the electrodes (2f, 2g) formed on the semiconductor light emitting element (2) to the pair of external terminals (3, 4); pouring the coating agent into the concavity (3a) to cover the semiconductor light emitting element (2), the electrodes (2f, 2g) and the end portions of the lead wires (5, 6) connected to the electrodes (2f, 2g) by the coating agent, the coating agent comprising a metal alcoxide or a ceramic precursor polymer, having a permeability to the light projected from the semiconductor light emitting element (2), and containing a fluorescent substance which absorbs the light projected from the semiconductor light emitting element (2), and converts it into light different in emission light wavelength; baking the coating agent at a temperature lower than the melting point of the semiconductor light emitting element (2) to form the coating material (10) which tightly and strongly adheres to the semiconductor light emitting element (2) and the external terminals (3, 4); and finally sealing the coating material (10) with the plastic encapsulant (8).